Via auto update or drop by the Zimbra Desktop Downloads page.

Highlights:
-Fixed searching with exact quotes in index. (Reindex via setup if you wish to apply on existing content.)
-Zimlets: Socalcast.com in Social tab, Twitter certificate adjusted, custom installed Zimlets remain on upgrades.
-When you modify of a recurring series appointment you are now prompted about what to do with exceptions (previously left them alone, however doing so can actually cause trouble in mixed environments.)
-Trash/synced last to get your important content first.
-Corrections for Gmail, Aol, Yahoo! accounts.
-More details in the release notes.

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If first time using ZD 7.x series:
-Coming from prior ZD/ZCS versions you'll find this doc a good read, lots of nifty UI tips with a very detailed explanation of the newest features in 7.x.
-When using Yahoo! Contacts and this is your first upgrade to ZD 7.x from ZD 2.x you'll have to edit the account briefly to make sure OAuth verification get's setup properly.